A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for the traditional internal-combustion engine used in modern vehicles.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode layers to form a membrane-electrode-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen from a hydrogen source and oxygen from an air stream, for an electrochemical fuel cell reaction. In automotive applications, individual fuel cells are often stacked together in series to form a fuel cell stack having a voltage sufficient to power an electric vehicle.
To maximize an operating efficiency and an amount of electricity produced, it is desirable for the fuel cell to be properly humidified. Over-humidifying the fuel cell can result in an excessive formation of liquid water that impedes the migration of the gaseous reactants to the electrodes, and minimizes the production of electricity. Under-humidifying the fuel cell can dry out the MEA and may limit the proton transport required in the electrochemical fuel cell reaction.
At least one of the hydrogen and the air stream is typically humidified by one of several methods known in the art. For example, in U.S. Pat. No. 6,376,111, hereby incorporated herein by reference in its entirety, a controller utilizes feedback to control the humidity of a fuel cell assembly. A resistance of the fuel cell assembly measured across a converter is used to control the humidity of the fuel cell assembly.
Relative humidity sensors are generally used to measure and control the humidity level in the fuel cell system. Commercially available humidity sensors, such as capacitive sensors with hydrophilic dielectric materials used to convert water vapor concentration into an electric signal, have been used to obtain the relative humidity readings from fuel cell reactant supply conduits. Upon a shut-down of the fuel cell stack, however, a temperature within the conduits is lowered and may reach a dew point. Upon reaching the dew point, liquid water condenses within the conduit and on the humidity sensors. The liquid water on the humidity sensors leads to inaccurate humidity readings or short-term “blinding” upon start-up of the fuel cell system. The exposure to liquid water may also reduce the useful life of the humidity sensors, for example, due to corrosion or a swelling of the hydrophilic components.
To address the known problems of humidity sensors in fuel cell applications, humidity sensors having an optimized corrosion resistance have been employed. Additionally, heating elements have been used in the oxidant conduits to heat an area around the humidity sensors, thereby militating against a formation of liquid water. These solutions have not been desirably effective, however, in optimizing a durability and an accuracy of humidity sensors in fuel cell systems.
There is a continuing need for a humidity sensing device for a fuel cell system that optimizes a durability and an accuracy of the humidity sensor. Desirably, the humidity sensing device militates against a short-term blinding and a long-term corrosion and swelling of the humidity sensor.